Jason's Borger has posted the difference between a standard overhead cast and a roll cast in terms of rod "butt" rotation. Take a look at his blog for other casting articles. Jason's Blog:

Graph from Jason's Blog, the top graph is the stardard cast and the bottom is the roll cast.:

The difference between the two graphs is that the final (accelertion) rod loading for a roll cast must occur over a shorter time and have a higher peak. It requires more energy to make a roll cast because part of the energy is used to both elevate the line and to break the line free of surface tension.

The standard casting instruction for the roll cast has been that it is just the same as the a regular forward cast. We can see that that is not quite correct. If you look at the white side of the graph, the initial front part of the curve (from about 37 to 55 ms) is identical UNTIL the final steep acceleration from 55 to 58 ms. So the final application of power is more sudden and forceful than an in the air cast.

The other thing to notice is that even though the final acceleration is more sudden and rapid, it is still SMOOTH. There are no dips in the acceleration line.

As a geek, I found the graphs very interesting. I never realized that there was a difference between a roll cast and a standard cast in terms of casting dynamics. We are often told that the roll cast is just 1/2 of a standard cast and they are identical. Apparently there are differences that I think are significant, and it explains to me why beginners find a good roll cast to be difficult.

If I remember my physics correctly:

I would guess the higher peak and more rapid angular acceleration is needed in a roll cast, because a greater force over time is required to both lift the line up against the pull of gravity and to break the line free of surface tension. Force over time is work, so the forward roll cast takes more work than the forward half of an in-the-air cast.

If I remember correctly, work should be the area under the curves for both these graphs. Work is the integral of force over time. But the difference in not only the total amount of work done for each cast.

The graphs also show that the main application of the angular acceleration is contracted into a shorter time frame for the roll cast than with the in-the-air cast. I don't know if this is true, but this suggest to me that timing is more critical with a roll cast than an in the air cast. This is another way of saying that the acceleration is smoother with the in-the-air cast, and the roll cast requires the extra "umph" at just the right time and in just the right amount.

Don't know if I am right about my musings. We need a professor of physics to sort it all out."